1. Field of the Invention
The present invention relates to a cerium-based oxide fiber and its fabricating method, and more particularly to a cerium-based oxide fiber and its fabricating method of which a chemical modifier is added to a cerium-based nitrate solution for a synthesis by aging below the boiling point of water from 10 hours to 50 hours, and the fibers have a diameter from submicron to micron size, and the aspect ratio of the fibers is greater than 100, and hydrate fibers can be converted to oxide fibers after going through a calcination at a high temperature.
2. Description of the Related Art
Cerium oxide (CeO2) has the same crystalline structure of calcium fluoride (CaF2); its atomic packing configuration is a cubic crystal series; the coordination number of the cerium ion is 8; the crystal lattice constant of cerium oxide is 0.5411 nm; its space group is SFm 3m; and the density of highly pure cerium oxide is 7.215 g/cm3.
Since cerium oxide features high reactivity, high activity, and special optoelectronic properties, therefore cerium oxide have extensive applications in many areas. In addition to its traditional applications as a polisher for polishing glass, a catalyst for a catalyst converter in a fuel engine exhaust pipe and a solid electrolyte in a solid oxide fuel cell, cerium oxide also can be used in semiconductor processes in recent years. Since the hardness of cerium oxide powder is appropriate for semiconductor processes, ceramic oxide plays an important role in a chemical mechanical planarization (CMP) polishing process. However, these applications require granule powder of nano level or micron level.
The methods for preparing traditional cerium oxide powder generally include a co-precipitation method and a hydrothermal method, wherein the co-precipitation method adds two or more cation solutions into a precipitating agent to obtain a precipitate with an even composition which can be synthesized into a metal compound of different compositions. The features of the co-precipitation method include controllable particle size and shape. The surface of produced nano powder is highly active, and the co-precipitation method also produces particles with an even diameter and involves simple and easy equipment operations and low costs. However, the drawbacks of the co-precipitation method include its frequent formation of colloidal precipitates which are difficult to filter, and it is necessary to rinse the precipitate several times before removing anion impurities.
The hydrothermal method reacts a solution under a high temperature and a high pressure to assist the growth of the crystalline phase of particles, and its advantage resides on the formation of crystalline grains of a certain grain level and crystalline configurations at a temperature much lower than the calcination temperature which induced sintering of the fine powder. However, the hydrothermal method requires a high-pressure reactor and incurs a high cost, and this method cannot be used for continuous productions. Table 1 summarizes the literatures regarding the cerium oxide powder in recent years and also points out the detailed basic properties of the powders, including the shape, size, and aspect ratio of the powders.
There were many literatures describing the electric properties of cerium oxide in the past, and if a trivalent or quadrivalent cerium ion is added to an oxide lower than the quadrivalence (such as trivalence), an oxygen vacancy will occur. Thus, the electric conductivity of cerium oxide depends on the concentration of oxygen vacancy. Further, cerium has a certain level of electric conductivity. In the report of Tschöpe [“Interface Defect Chemistry and Effective Conductivity in Polycrystalline Cerium Oxide”, Journal of Electroceramics, 14, (2005) 5-23], we understand that the crystal size of cerium oxide will affect the ratio of electric conductivities of two ionic species. The smaller the crystalline grain, the stronger is the electric conductivity and the lower ratio of electric conductivities of ions. The main reason is that the electric conductivity of the ion is operated according to oxygen vacancy.
Some silicon impurities may be segregated at the grain boundary of cerium oxide in a synthesis, and thus the boundary impurities will lower its electric conductivity. Among the present cerium oxide additives, gadolinium (Gd) is the most popular one because the size of gadolinium ions is very close to the size of cerium ions, and the valence of gadolinium ions is smaller than the valence of cerium ions, and thus it almost has no stress remained in crystal lattices, and a defect association will not occur at the grain boundary under a low-temperature operation. In other words, the oxygen vacancy will not be fixed, and a very good ionic conductivity for the oxygen-deficient ions at a low temperature of 400° C.˜600° C. can be obtained.
However, the precipitates produced by the foregoing co-precipitation method are usually in a colloidal form and cannot be filtered easily, and these precipitates must be rinsed several times before the anion impurities can be removed. The hydrothermal method requires a high-pressure cylinder reactor, and incurs a high cost, but it still cannot be used for continuous productions.
Therefore, the present invention provides a cerium-based oxide fiber and its fabricating method that add a modifier to cerium nitrate solution and go through a water based acid-base reaction to hold the temperature below the boiling point of water and grow cerium-based hydrate fibers in a range from 10 hours to 50 hours. The diameter of these fibers falls into a range from submicron to micron, and the fiber diameter is very even, and the aspect ratio exceeds 100. After this fiber goes through the calcination process, hydrate is converted into oxide, and the fiber remains its fibrous form.